GNU Make integration: Correctly react to the presence of `n` (`--just-print`) in `MAKEFLAGS`

Problem

Thanks to cargo implementing the jobserver protocol (#1744), you can already call cargo from a GNU make build and have both build systems figure out how many parallel jobs can be started at a given time. All you have to do is prefix the GNU Make recipe line in which cargo is invoked with a +.

Unfortunately, this has a nasty side effect: If you (or your IDE [^1]) only want to know what GNU Make would execute and start make -n, then make executes cargo anyway, assuming that sub-makes will know how to handle the MAKEFLAGS appropriately. Unfortunately, cargo actually completely ignores the presence of n in MAKEFLAGS and starts building stuff, which is the exact opposite of what the user wanted.

[^1]: CLion, for example, does this when importing Makefile projects

Proposed Solution

If n is present in MAKEFLAGS, cargo should only print the processes it would normally execute instead of actually executing build processes.

Notes

Relevant GNU Make documentation:

• Instead of Execution (GNU make)
• MAKE Variable (GNU make)

Start by tracing Cargo's handling of MAKEFLAGS and its GNU Make jobserver integration, then reproduce the issue with make -n and a recipe invoking cargo. The work is done when a MAKEFLAGS value containing n causes Cargo to print the processes it would execute without starting build processes; add or update coverage for this behavior.